The invention is generally related to floating offshore structures and more particularly to the centerwell arrangement of a spar type hull.
There are a number of spar hull designs available in the offshore oil and gas drilling and production industry. These include the truss spar, classic spar, and cell spar. The term spar hull structure described herein refers to any floating structure platform, which those of ordinary skill in the offshore industry will understand as any floating production and/or drilling platform or vessel having an open centerwell configuration.
A spar hull is designed to support a topsides platform and riser system used to extract hydrocarbons from reservoirs beneath the seafloor. The topsides support equipment to process the hydrocarbons for export to transport pipelines or to a tanker for transport. The topsides can also support drilling equipment to drill and complete the wells penetrating the reservoir. The product from these wells is brought up to the production platform on the topsides by risers. The riser systems may be either flexible or steel catenary risers (SCRs) or top tensioned risers (TTRs) or a combination of both.
The catenary risers may be attached at any point on the spar hull and routed to the production equipment on the topsides. The routing may be on the exterior of the hull or through the interior of the hull. The TTRs are generally routed from wellheads on the seafloor to the production equipment on the topsides platform through the open centerwell.
These TTRs may be used for either production risers to bring product up from the reservoir or as drilling risers to drill the wells and provide access to the reservoirs. In some designs where TTRs are used, either buoyancy cans or pneumatic-hydraulic tensioners can support (hold up) these risers. When buoyancy cans are used, the buoyancy to hold up the risers is supplied independently of the hull and when tensioners are used these tensioners are mounted on the spar hull and thus the buoyancy to hold up the risers is supplied by the spar hull. In either method of supporting the risers, TTRs are generally arranged in a matrix configuration inside an open centerwell. The spacing among the risers in this centerwell location is set to create a distance among the risers that allows manual access to the production trees mounted on top of the risers.
The spar type structure which supports the topsides comprises a hard tank and other structural sections such as a truss and a soft tank or the hull can be completely enclosed as a cylinder. The hard tank supplies the majority of the buoyancy to support the hull structure, risers, and topsides platform. The hard tank is compartmentalized into a plurality of chambers among which the ballast can be shifted to control the hull's stability.
The centerwell configuration forms an open volume in the center of the hard tank referred to as the open centerwell. Since the centerwell is open to the sea it does not contribute to the hull structure's buoyancy. This offers a potential to displace the sea water in the centerwell and capture the buoyancy. The primary advantage of capturing this buoyancy is that the diameter of the hard tank can be reduced. This offers specific benefits in construction, transportation and installation of the spar hull.